Ammonium phosphate suitable for use as fertilizers can be obtained from wet-process phosphoric acids simply by neutralization with ammonia. It is often necessary, if not essential, however, to increase the P.sub.2 O.sub.5 content, i.e., reduce the H.sub.2 O/P.sub.2 O.sub.5 ratio, of the wet-process acid prior to neutralization. For instance, it may be desirable to produce an end product having a P.sub.2 O.sub.5 content higher than that which can be obtained directly from available feeds.
Secondly, numerous processes have been devised to take advantage of the autogenous heat of neutralization of phosphoric acid with ammonia and achieve at least some polymerization to polyphosphates. This approach has several advantages. Firstly, wet-process acids contain substantial amounts of metallic and non-metallic impurities. These impurities promote precipitation or gelation upon neutralization with ammmonia unless the solution contains sufficient polyphosphates.
Polyphosphates are generally not found in wet-process acid feeds as obtained. They can be formed, however, by heating the raw or partially neutralized acid to relatively high temperatures, e.g., in excess of 260.degree. C. At temperatures above about 260.degree. C. sufficient water is driven from the solution to promote condensation of the phosphate groups into polymers, higher polymer or conversion levels being achieved at higher temperatures.
Substantial heat loads are required to accomplish this objective, and obviously more heat is required to drive off greater volumes of water. Thus, depending upon the H.sub.2 O/P.sub.2 O.sub.5 molar ratio of the available acid feed, it may be impossible to reach polymerization temperatures by adiabatic neutralization alone. It thus becomes necessary to preconcentrate the acid and reduce the H.sub.2 O/P.sub.2 O.sub.5 ratio to a level sufficient to obtain the required temperatures by neutralization. For that matter it is sometimes desirable to concentrate the feed acid to a point sufficient to form some polymeric phosphates with or without post neutralization.
In any event, evaporative preconcentration requires evolution in substantial water vapor. The expelled vapor carries with it some amounts of phosphoric acid, fluorine and silicon compounds. Fluorine and silicon are generally present in the vapor phase as hydrogen fluoride, silicon tetrafluoride and fluorosilicates.
These compounds, either alone or in combination with the phosphoric acid and/or water, significantly complicate vapor effluent recovery. For ecological and safety reasons, they can not be released to the atmosphere. Thus the prior art reports numerous attempts to solve this problem. For instance, U.S. Pat. No. 3,714,330 discusses the use of scrubbers and points out that such apparatus is very expensive due to the difficulty of recovering the aerosols or smokes produced by the vapor phase. Such apparatus is also subject to corrosion due to the acidic nature of the phosphoric acid, hydrofluroic acid and silicon tetrafluoride. Similarly, as pointed out in the U.S. Pat. No. 3,193,351, the recovery of these aerosols or smokes by scrubbing is very inefficient. This is obviously due largely to the physical and/or chemical nature of the aerosol formed in those processes.
These difficulties have led to several attempts to solve the problem of vapor phase recovery by other approaches. For instance, U.S. Pat. No. 3,764,657 suggests that the impurities which are apparently responsible for aerosol production can be removed with organic solvents such as the lower alcohols and ketones. However, this process requires the use of expensive solvents, preliminary scrubbing and separation steps, solvent recovery and purification. U.S. Pat. No. 3,800,029 suggests that at least some of the fluorine and silicon can be removed by precipitation as sodium fluorosilicate upon the addition of certain sodium compounds. This method also requires additional materials and process steps.
It is therefore one object of this invention to provide an improved method for concentrating wet-process phosphoric acid. It is another object to provide a method for producing ammonium phosphates including a wet-process acid preconcentration step with an improved, simplified method of recovering the vapor phase from the evaporator. Another object is the provision of an improved method for recovering the vapor phase generated by the evaporative preconcentration of wet-process acids containing silicon and fluorine compounds. Another object is the provision of an improved method of producing ammonium phosphate and purified wet-process phosphoric acids.